This invention relates to tuning apparatus for a radio receiver and, more particularly, to such apparatus for use with a frequency-synthesizer type arrangement wherein the tuning condition thereof is accurately controlled so as to receive broadcast frequencies which are separated from each other by relatively small frequency steps, and wherein the tuning section is tuned accurately to a proper broadcast frequency regardless of whether the broadcast signal is a relatively high level or low level signal.
Frequency-synthesizer tuning arrangements have been proposed for FM radio receivers. In such arrangements, the oscillating frequency of a variable oscillator, such as a voltage controlled oscillator (VCO) is determined by a digital number, or count, representing a particular broadcast frequency to which the tuning arrangement is to be tuned. As this digital count changes, the tuning condition of the FM radio receiver likewise changes. The broadcast spectrum can be scanned by the radio receiver merely by incrementing (or decrementing) the digital count, as desired.
Heretofore, such frequency-synthesizer tuning arrangements have been used primarily in FM radio receivers, but not in AM receivers. This is because the frequency separation of adjacent FM broadcast frequencies is relatively high, but AM broadcast frequencies are closely spaced from each other. For example, in the United States, the FM frequency band is within the range of 88 MHz to 108 MHz, with each broadcast channel having a bandwidth of 200 KHz. Hence, to avoid overlapping of adjacent FM broadcast channels in a particular region, adjacent FM broadcast frequencies are separated from each other by multiples of 100 KHz (e.g. adjacent FM broadcast frequencies may be 400 KHz apart). Because of this relatively wide separation between adjacent FM broadcast frequencies, there generally is little difficulty in determining when a true broadcast frequency is received and in precisely tuning the radio receiver to that FM frequency.
However, in the United States, the AM frequency band extends over the range of 535 KHz to 1605 KHz with each broadcast channel having a bandwidth of about 10 KHz (in other countries, such as in Japan, each AM broadcast channel has a bandwidth of 9 KHz). Hence, adjacent AM broadcast frequencies are separated by multiples of 10 KHz (9 KHz in Japan), such that adjacent AM broadcast frequencies may be 20 KHz apart (18 KHz in Japan). Such a narrow bandwidth and close spacing of adjacent AM broadcast frequencies have presented problems in designing frequency synthesizer arrangements for AM radio receivers which are capable of being tuned correctly to a true AM broadcast frequency.
One difficulty in using a frequency synthesizer arrangement in the tuning section of an AM radio receiver is that during the initial "set-up" phase wherein the AM broadcast spectrum may be scanned in order to select desired AM broadcast frequencies for future tuning, such broadcast frequencies may not be received accurately. Typically, an indication that the radio receiver is tuned to a broadcast frequency is derived from the IF detector which normally is provided in the IF section of that receiver. That is, it is assumed that the radio receiver is tuned to a broadcast frequency if the IF signal level exceeds some predetermined threshold level. If the frequency synthesizer scans the AM broadcast spectrum, it is expected that the IF signal level will exceed this threshold level only when the frequency synthesizer arrangement is tuned to a respective broadcast frequency. The scanning operation may be halted at each such indication. However, the setting of the threshold level must be made very carefully. If this threshold level is too high, and if the broadcast signals are relatively low level signals, then even when the frequency synthesizer arrangement is tuned accurately to a broadcast frequency, the signal level of the IF signal may not excced this high threshold. Consequently, the scanning operation will not be halted, and desired AM broadcast channels will not be received. Conversely, if the threshold level is too low, the IF signal level may exceed this threshold level even when the frequency synthesizer is not yet tuned to the proper broadcast frequency. For example, the tuning condition of the frequency synthesizer may differ from the proper broadcast frequency by a small frequency step .DELTA.f; yet the IF signal which is produced at this tuning condition may have a signal level which exceeds the threshold level. Hence, the scanning operation will be halted even though the radio receiver is not yet tuned to the proper broadcast frequency.
In view of the relatively narrow bandwidth of each AM broadcast channel, and further in view of the close spacing of adjacent AM broadcast frequencies, the aforementioned difficulties are far more serious in AM radio receivers than in FM radio receivers.